In a large skillet, cook beef over medium heat until no longer pink; drain. Stir in the chili powder, cumin, coriander, salt and oregano.
Combine cornstarch and water until smooth. Stir into beef mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with taco shells. Garnish with toppings if desired. Yield: 4 servings.